Such display mechanisms are well known to those skilled in the art. They generally comprise a display member which is fitted with a toothing, mobile in rotation, and driven by a wheel set arranged for delivering a brief impulse thereto, upon the passage from one time indication to the next. The wheel set is conventionally formed of a finger that cooperates with the toothing to move the latter forward one step, a cam secured to the finger in rotation, and a drive wheel which draws its drive force from a movement.
An example of this type of instantaneous display mechanism is illustrated in FIG. 1. The mechanism includes, in a conventional manner, a display member 10, formed of a date ring 12 fitted with a toothing 14, mounted to move in rotation on a plate that is not shown. The display member 10 is positioned angularly using a jumper spring 16. It is actuated in rotation by a wheel set 18, which is itself driven in rotation clockwise by an intermediate wheel 20, meshed with an hour wheel 22.
Wheel set 18 includes a finger 24 and a cam 26 secured in rotation to finger 24 and oriented angularly relative to finger 24. For this purpose, cam 26 is fitted with a stud 28 engaged in a cut out portion 30 of finger 24. Wheel set 18 further includes a drive wheel 32, meshed with intermediate wheel 20, in which an aperture 34, in the form of an arc of a circle, is made, comprising an active end 34a and a passive end 34b. Stud 28 passes through aperture 34, such that the assembly of finger 24-cam 26 forms a coupling with play, wherein the finger 24-cam 26 assembly is free to pivot at a certain angle. A lever 36 abuts on cam 26 via the action of a spring 38.
Cam 26, illustrated in FIG. 2, includes a portion 26a of increasing radius, a portion 26b of rapidly decreasing radius and a portion 26c of minimum radius. The various portions define, for a given point of abutment of lever 36, different angular positioning sectors of finger 24, corresponding to distinct advancement modes of the finger 24-cam 26 assembly. The first two portions 26a and 26b define two angular sectors, respectively BA and AB, delimited by two angular positions A and B of finger 24. In position A, finger 24 is oriented upstream of but released from toothing 14 and in position B, finger 24 is oriented downstream of but engaged in toothing 14. It should be noted that finger 24 is said to be “engaged” in toothing 14 if it is located on the path of toothing 14, and “released” from toothing 14 if it is located outside the path of toothing 14. The last portion 26c defines a stable position of the finger 24-cam 26 assembly, wherein finger 24 is in position B.
Over the first angular sector BA, the finger 24-cam 26 assembly is driven in rotation by drive wheel 32, via the action of stud 28 which cooperates with the active end 34a of aperture 34. Over this entire angular sector, lever 36 abuts on portion 26a, such that spring 38 is gradually strained via the effect of the increase in radius of cam 26.
When finger 24 reaches position A, the point of abutment of lever 36 passes from portion 26a to portion 26b. Via the effect of the rapid decrease in radius of cam 26, the spring abruptly slackens, driving in rotation the finger 24-cam 26 assembly over the entire second angular sector AB.
After pivoting, finger 24 is in position B and lever 36 is abutting on portion 26c, which corresponds to the minimum strain position of spring 38. The finger 24-cam 26 assembly is in a stable position. This configuration is illustrated in FIG. 3. Stud 28 is now located at about two thirds of the way along aperture 34, such that drive wheel 32 stops driving the finger 24-cam 26 assembly during the period of time necessary for active end 34a of aperture 34 to catch up with stud 28. This period of time is typically of the order of 8 hours.
The abrupt passage of finger 24 from position A to position B has driven display member 10 through one step in rotation. For approximately 8 hours after this rotation of display member 10, finger 24 is locked in position B via the effect of spring 38, which holds lever 36 abutting against portion 26c. In this position, it has been observed that there is a risk of poor manipulation causing significant damage.
Indeed, the time indication display mechanism is generally provided with a quick correction device for driving display mechanism 10 directly in its current operating direction, namely clockwise in this case. When the user actuates the quick correction device while finger 24 is in position B, engaged in toothing 14, display member 10 drives finger 24 in rotation, via toothing 14. It will be noted that this manipulation is possible since stud 28 is not stopped against the passive end 34b of aperture 34, but at a sufficient distance for finger 24 to be able to be released from toothing 14 without being blocked by drive wheel 32.
If the quick correction device is partially actuated, finger 24 may be oriented in a singular position referenced S. In this position S, finger 24 is abutting on the tip of toothing 14, on which it exerts a significant force because of the action of spring 38, which holds lever 36 abutting on portion 26a of increasing radius of cam 26. Display member 10 is locked in an intermediate position, since the action of jumper spring 16, which tends to make it rotate, is not enough to counter the action of spring 38, which is locking said member This configuration is illustrated in FIG. 4.
Since finger 24 is positioned in singular position S, the user may wish to go backwards, if he observes that he should really have corrected the display in the opposite direction of rotation to the current operating direction. Generally, quick correction devices do not allow quick correction in the opposite direction of rotation, because of the high risk of breaking the display mechanism. For this reason, they are provided with disconnecting means or sliding gear means for switching the correction of one display mechanism to another, depending upon the direction of rotation However, correction in the opposite direction to the operating direction is possible via correction of the time. If, therefore, the user tries to correct the position of display member 10 in the opposite direction of rotation to the current operating direction, while finger 24 is immobilised in singular position S, this manipulation will inevitably lead to the breakage of finger 24 or part of toothing 14. The instantaneous display mechanism is then no longer operational.